Power transistors are commonly used in many radio frequency (RF) devices and circuits including those incorporated into cellular radio telephones and cellular radio base station transceivers. Often, the stability of the output power is critical to the correct operation of the device or circuit, not only to ensure accurate RF output power levels but also to prevent damage to transistor itself. A common cause of output power instability is variation of the operating temperature of the power transistor, such as can arise due to self-heating and changes in the ambient temperature.
In order to ensure that a power transistor amplifies an RF signal with a desired gain, it is necessary to bias the transistor at a suitable "operating point". A simple biasing arrangement is shown in FIG. 1, where an RF signal RF.sub.-- IN to be amplified is coupled to the input of a power transistor Q1. The amplified RF signal RF.sub.-- OUT is obtained from the collector of the power transistor. A driver transistor Q2 is also coupled to the input of the power transistor Q1 to bias the power transistor at a suitable operating point. Temperature changes will however tend to change the base-emitter voltages of both the power and driver transistor, changing the operating point and therefore gain of the power transistor Q1. Provided that the emitter resistance Re is large, an increase in gain and collector current will tend to be offset by a decrease in the base-emitter voltage introducing a degree of stability to the transistor. However, a large emitter resistance results in a large power loss across that resistance, decreasing the efficiency of the amplifier.
FIG. 2 illustrates a modified power amplifier incorporating a typical temperature compensating biasing circuit. The circuit comprises a pair of diode-connected transistors Q3, Q4 connected between the base of the driver transistor Q2 and ground. The underlying operating theory is that the voltage drop across the two diode-connected transistors Q3, Q4 tracks that across the base-emitter junctions of the driver and power transistors Q1, Q2, maintaining the power transistor base current substantially constant and its operating point stable. As with the circuit of FIG. 1, additional stability is provided by the voltage drop across the emitter resistance Re although again this tends to reduce the efficiency of the amplifier.
Although the temperature compensation circuit of FIG. 2 is widely used, it is not ideal. Firstly, fluctuations in the bias voltage Vbias, usually generated from a regulated voltage (e.g. the supply voltage Vsup), have a significant effect on the collector current Ic of the power transistor Q1. Secondly, as the base-emitter regions of the two diode-connected transistors Q3, Q4 are not operating under exactly the same conditions as the power and driver transistors Q1, Q2 (for example they do not receive the RF input signal), the voltage drop across the former may not necessarily follow that across the base-emitter junctions of the latter, even if the operating temperatures of all four transistors are substantially identical. This problem is exacerbated by differences between the various transistors which arise from possibly wide manufacturing tolerances, even where the entire circuit is integrated onto a single semiconductor chip.